1. Field of the Invention
The present invention relates to a control system for controlling engaging and disengaging operations of a releasable coupling device such as a computer-controlled clutch placed in an automotive power train between an engine crankshaft and drive wheels automatically to couple the engine and the transmission at least during vehicle travel and to uncouple them at least during vehicle stand-still, or a computer-controlled, hydraulically-actuated lock-up clutch capable of engaging automatically to lock the impeller and turbine wheels together to give a direct drive at a predetermined ratio of turbine-to-engine speed.
2. Description of the Prior Art
In recent years, there have been proposed and developed various control systems for controlling engaging and disengaging operations of a releasable coupling device such as a so-called start clutch for a continuously variable transmission (often abbreviated to a "CVT") or a lock-up clutch employed in an automotive automatic transmission with a so-called lock-up torque converter. For example, Japanese Patent Provisional Publication Nos. 5-141526 and 8-21526 disclose systems for controlling the engaging and disengaging of a lock-up clutch. In the Japanese Patent Provisional Publication No. 5-141526, a control system operates to temporarily release a lock-up clutch in presence of the output of a brake signal resulting from depression of a brake pedal and to re-engage the lock-up clutch, when, after a predetermined time period has been elapsed from the temporary release, a vehicle deceleration is still below a predetermined value or a vehicle speed is still above a predetermined speed. This re-engagement of the lock-up clutch, being executable during moderate deceleration of the vehicle or during driving at a medium or high speed greater than the predetermined speed, ensures a better engine braking effect (a better engine braking response) or a better exhaust braking effect (a better exhaust braking response). On the other hand, the Japanese Patent Provisional Publication No. 8-21526 teaches the setting of engagement capacity of lock-up to the minimum acceptable lock-up engagement capacity just below the point where the transmission torque converter would internally slip owing to back torque-flow directed from drive wheels towards the engine during coasting of the vehicle (during vehicle inertial travel owing to vehicle inertia without any driving torque (positive wheel torque) transmitted to drive wheels). Thus, when shifting from such coasting to quick vehicle deceleration (owing to so hard braking), the lock-up clutch can be abruptly released, thus reducing risk of stalling the engine. The system disclosed in the Japanese Patent Provisional Publication No. 8-21526 operates to keep lock-up at the previously-noted minimum acceptable lock-up engagement capacity if the magnitude of vehicle deceleration monitored is less than a predetermined threshold value during shifting from coasting to quick deceleration. A system for controlling an automatic clutch sometimes called a "start clutch", different from the previously-discussed lock-up clutch, has been disclosed in Japanese Patent Provisional Publication Nos. 7-229525 and 8-226501.
Of these prior art control systems discussed above, for instance when taking notice of a control system for a lock-up clutch, as disclosed in the Japanese Patent Provisional Publication No. 8-21526 assigned to the assignee of the present invention, an electronic control unit generally uses a throttle opening and a vehicle speed as control parameters for automatically controlling the lock-up clutch. The vehicle speed can be arithmetically calculated from revolution speeds of the output shaft of a transmission and be regarded as a value equivalent to a rotational speed of drive wheels. Usually, when the vehicle speed (equivalent to a drive-wheel speed) exceeds a predetermined value during coasting, the lock-up clutch is brought into engagement. Conversely, as soon as the vehicle speed becomes below the predetermined value, the lock-up clutch is released. In the conventional control system, a shifting action from one of application (engagement) and release (disengagement) of the lock-up clutch to another would be attained abruptly for a too brief moment. In other words, lock-up is automatically turned on or off for a too brief moment in response to a control command signal from the electronic control unit (ECU). Abrupt application or release of the lock-up clutch results in several drawbacks. For example, when the ECU abruptly releases the torque-converter lock-up feature, the driver may experience an uncomfortable acceleration feel. Such an uncomfortable acceleration feel caused by abrupt release of the lockup clutch, is remarkable on automotive vehicles equipped with an electronically-controlled engine having a fuel cutoff unit which is able to operate at a so-called coast fuel-cutoff mode to temporarily shut off fuel supply to fuel injectors during coasting of the vehicle in order to insure a better engine braking response and to improve fuel economy. That is, when the vehicle is decelerated rapidly during severe braking, the drive-wheel speed will soon become below the predetermined threshold value, and thus the lock-up clutch is released abruptly. At the same time, the fuel cutoff unit begins to operate at the fuel cutoff mode. As a consequence, transmission of back torque-flow from the drive wheels to the engine is shut off with the lock-up clutch disengaged and also the engine speed drops owing to deceleration fuel cutoff. This results in a rapid drop in the engine speed. To avoid engine stall tendencies due to rapid drop in the engine speed, the ECU switches from the fuel cutoff mode to a fuel-supply recovery mode (simply fuel-recovery mode), with the result that the vehicle begins to accelerate again with abruptly increased engine output power. This may amplify uncomfortable feeling of the driver during lock-up and fuel cutoff controls.